Question

The heat is liberated when 1.89 g of benzoic acid is burnt in a bomb calorimeter at ${25}^{0}C$ and it increases the temperature of 18.94 kg of water by ${0.632}^{0}C$. If the specific heat of water at ${25}^{0}C$ is $0.998cal/g.deg$, then the value of the heat of combustion of benzoic acid is :

• A. $881.1$ kcal
• B. $981.1$ kcal
• C. $771.1$ kcal
• D. $871.2$kcal

Answer 1

The correct option is C $771.1$ kcal

Mass of benzoic acid = 1.89 g

Temperature oof bomb calorimeter= ${25}^{0}C=298K$

Mass of water (m) =18.94 kg=18940 g

Increase in temperature $\left(\Delta t\right)={0.632}^{0}C$

Specific heat of water (s) = 0.998 cal/g-deg

Heat gained by water or heat liberated by benzoic acid$\left(Q\right)=ms\Delta t$=$18940\times 0.998\times 0.632=11946.14cal$
Since, 1.89 g of acid, liberates 11946.14 cal of heat.

therefore heat liberated by 122 g (mol.wt of benzoic acid) of acid =$\frac{11946.14\times 122}{1.89}=771126.5cal\approx 771.1kcal$

Hence, the correct option is C.